JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
AMERICAN WATER RESOURCES ASSOCIATION
THREATS TO RIPARIAN ECOSYSTEMS IN WESTERN
NORTH AMERICA: AN ANALYSIS OF EXISTING LITERATURE1
Boris Poff, Karen A. Koestner, Daniel G. Neary, and Victoria Henderson2
ABSTRACT: A total of 453 journal articles, reports, books, and book chapters addressing threats to riparian ecosystems in western North America were analyzed to identify, quantify, and qualify the major threats to these
ecosystems as represented in the existing literature. Publications were identified either as research, policy, literature review, historical comparison, or management papers. All papers were evaluated based on year of publication, area of interest, and type(s) of threats addressed. Research papers, however, were assessed in more depth.
The publications ranged from the 1930s to 2010 and addressed the following threats: dams, pollution (point and
nonpoint), grazing, land use change, timber harvesting, water diversion, road construction, recreation, mining,
groundwater pumping, invasive species, climate change, salinity, fire, insect and diseases, woody encroachment,
watershed degradation, elimination of native vegetation, beavers, fire suppression, and fuel management. While
the types of threats vary on spatial and temporal scales, some persist through decades in western North
America. This analysis shows that grazing has been perceived as a dominant threat since the 1980s, but has
been diminishing in the past decade, while invasive species, dams and, in recent years, climate change are
increasingly represented in the literature as threats to riparian ecosystems in western North America.
(KEY TERMS: environmental impacts; riparian ecology; sustainability; water policy.)
Poff, Boris, Karen A. Koestner, Daniel G. Neary, and Victoria Henderson, 2011. Threats to Riparian Ecosystems
in Western North America: An Analysis of Existing Literature. Journal of the American Water Resources
Association (JAWRA) 1-14. DOI: 10.1111/j.1752-1688.2011.00571.x
threatened and endangered species are riparian obligates (Johnson, 1989). These ecosystems are not only
unique because they have high species diversity and
densities as well as high productivity, but they also
allow for continuous interactions to occur between
riparian, aquatic, and upland terrestrial ecosystems
through exchanges of energy, nutrients, and species
(Johnson and McCormick, 1978). Riparian ecosystems
are systems with a high water table because of
proximity to an aquatic ecosystem or subsurface
INTRODUCTION
Riparian ecosystems provide essential ecological
functions, especially in western North America. While
western riparian ecosystems occupy <2% of the total
land area (Svejcar, 1997), they provide habitat for
about one-third of the plant species. In the arid
Southwest about 60% of all vertebrate species
(Omhart and Anderson, 1982) and 70% of all
1
Paper No. JAWRA-10-0076-P of the Journal of the American Water Resources Association (JAWRA). Received May 7, 2010; accepted
May 5, 2011. ª 2011 American Water Resources Association. This article is a U.S. Government work and is in the public domain in the
USA. Discussions are open until six months from print publication.
2
Respectively, Hydrologist (Poff), Bureau of Land Management, Southern Nevada District Office, 4701 North Torrey Pines, Las Vegas,
Nevada 89130; and Hydrologic Research Technician, Science Team Leader, Biological Technician (Koestner, Neary, Henderson), Rocky
Mountain Research Station, USDA Forest Service, Flagstaff, Arizona 86001 (E-Mail ⁄ Poff: bpoff@blm.gov).
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responsible for these systems by providing a temporal
and spatial assessment of the types of threats which
scientists and policy makers have publicly discussed
over the course of two-thirds of a century.
water and have distinct vegetation and soil characteristics. Aridity, topographic relief, and presence of
depositional soils most strongly influence the extent
of high water tables and associated riparian ecosystems (Johnson and McCormick, 1978). Even though
riparian zones are not easily delineated, they are
composed of mosaics of landforms, communities, and
environments within the larger landscape (Gregory
et al., 1991).
Riparian zones range in width from a few to hundreds of meters. They can be found along ephemeral
stream channels, such as southwestern arroyos that
carry water only during rain or snowmelt wet periods, to perennial streams and floodplains, such as the
densely wooded banks of rivers in the Pacific Northwest (Obedzinski et al., 2001). Their vegetation varies
from aquatic graminiod-sedge wet meadows to extensive forests. The health of a western riparian ecosystem is usually measured in terms of watershed level
function, because of the ability to filter out large
quantities of sediments, nutrients, pesticides, animal
wastes, and other nonpoint source pollution (Obedzinski et al., 2001). Riparian vegetation influences light
penetration, air and water temperatures, and trophic
interactions as the transition between aquatic and
terrestrial zones. Large woody debris and litter associated with riparian vegetation are often necessary
for productive fish habitats, and influence the physical, chemical, and biotic characteristic of riparian and
instream ecosystems (Naiman et al., 1992). However,
in some riparian ecosystems, herbaceous plants provide the functions supplied by woody plants in other
locations (Baker et al., 2004).
In this article, we provide a description of the
major threats addressed in the literature, discuss the
type of literature we reviewed, quantify the threats
addressed therein, and look at the spatial and temporal distribution of these threats. Analysis will show
how some traditionally viewed threats, such as grazing, which according to the reviewed literature was
considered a dominant threat in the 1980s and 1990s,
have been diminishing in their extent, while other
threats, such as invasive aquatic and terrestrial
species, dams and, in recent years, climate change
are increasingly more pervasive in the scientific
literature.
In evaluating the impact of riparian threats, it is
important to comprehend the nature, dynamics, and
trends of past disturbances. Understanding and managing riparian systems requires recognition of the
role of disturbance and the evolutionary experience of
riparian organism within the context of regional ecosystems. If and when causes are understood, management changes that have real positive impacts can be
designed and implemented. Hence, the objective of
this paper is to aid managers and decision makers
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DESCRIPTION OF THE MAJOR THREATS
ADDRESSED IN THE LITERATURE
Grazing
There are more publications in the literature that
discuss grazing as a threat to western riparian ecosystems than any other single threat. However, as
livestock decreased across the West, expanding populations of native ungulates, particularly elk (Cervus
elaphus), have raised the potential to cause significant riparian impacts (Clary and Kruse, 2004). Ecological impact concerns have been identified in four
areas (Kauffman, 1988): (1) soil compaction, (2) herbage removal, (3) physical damage to plants, and
(4) changes in fluvial processes that may eliminate
germination sites for woody vegetation.
Herbage utilization by domestic or wild grazers
can influence the ecological character of vegetation by
altering structure and species composition (Kauffman
and Krueger, 1984). Generally, this change occurs
through the selective grazing habits, distribution,
and intensity of different types of herbivores, and the
sensitivity of individual plant species to animal traffic
and feeding. Kauffman and Krueger (1984) noted
that riparian overgrazing and site degradation
favored the replacement of native bunch grasses with
Kentucky bluegrass (Poa pratensis) in northeastern
Oregon. In many locations, the most important cause
for unsuccessful riparian plant reproduction was
grazing and trampling by domestic livestock (Ehrhart
and Hansen, 1997). Grazing of cottonwood seedlings
was reported to be a major factor limiting regeneration along a southern Arizona creek (Glinski, 1977).
In Yellowstone National Park, a 95% decline of tall
willows (Salix bebbiana, Salix boothii, Salix lutea,
and Salix geyeriana), and a virtual elimination of willow seed production was due to repeated elk browsing
(Kay and Chadde, 1991).
Cattle, sheep, and elk all have contributed substantially to geomorphic alterations of riparian ecosystems
(Medina, 1996). Grazing disrupts aggradation processes in riparian ecosystems through the cumulative
effect of herbivory and alteration of the vegetation and
soil matrix by hoof action (Neary and Medina, 1996).
The loss of riparian vegetation leads to a weakening of
streambanks and disruption of substrates that armor
channel bottoms and precipitates channel incisions.
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THREATS
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RIPARIAN ECOSYSTEMS
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WESTERN NORTH AMERICA: AN ANALYSIS
Riparian ecosystems provided areas for agriculture
for Native Americans before the arrival of European
settlers (National Academy of Science, 2002). With
the settlement of the West in the latter half of the
19th Century, riparian areas were extensively developed and altered. This continues today in smaller rivers like the Verde in Arizona (Tellman et al., 1997),
but also along larger rivers with broad alluvial
plains like the lower Colorado River in Arizona and
California (Johnson, 1978), the Sacramento River in
California (Spotts, 1988), and the Columbia River in
Oregon and Washington.
Urbanization, specifically, is a rather localized
threat to riparian ecosystems of major streams like
the Colorado, Rio Grande, Columbia, Snake, Missouri, and Sacramento rivers (Brinson et al., 1981;
Roelle and Hagenbuck, 1995; Braatne et al., 2008).
However, the associated threats produced by
increased human presence in the West produce
extensive and intensive impacts across the region
(Stromberg et al., 2004).
Dams
One of the consequences of dam construction that
provides both a threat and a benefit to riparian ecosystems is flow regulation. Dams have the effect of reducing peakflows for increased water storage, and
increasing or decreasing low base flows. These changes
in flow regime can have a substantial affect on both
riparian plants and aquatic biota. Crawford et al.
(1996) and Groeneveld and Griepentrog (1985)
reported declines in riparian vegetation in flow regulated rivers. Beauchamp and Stromberg (2007) measured no significant changes in Populus spp. and Salix
spp. stands on the Verde River of Arizona due to flow
regulation, but Tamarix establishment increased.
Beauchamp et al. (2006) noted marked declines in
herbaceous species richness and abundance due to
flow regulation. Fluctuating flows produced by flow
regulation for electricity generation and irrigation
have been known to reduce periphyton density in
rivers (Benenati et al., 1998).
Construction of a dam in Montana reduced the
magnitude and intensity of floods to the point that
cottonwood density decreased significantly over a 45year period (Bradley and Smith, 1986). In this case,
even though recruitment of cottonwood seedlings continues to occur, survival is low. The problem is that
not enough seedlings are growing in the critical zone.
This zone needs to be sufficiently above water level to
ensure rooting before high water disturbs seedling
establishment. It must also be near enough to the
surface water table to draw sufficient moisture to survive dry periods. Rood et al. (1995) found similar
results with dam construction on rivers in southern
Alberta.
Hadley and Emmett (1998) found the opposite to
be true with dam construction on a tributary of the
South Platte River. A dramatic shift in the riparian
plants from sparse grasses to dense woody vegetation
occurred after dam construction. This rapid change
was linked to channel scour, sediment deposition
declines, fewer scouring peakflows that damaged
woody plants, and an increase in dry season low
flows. In some parts of the West, riparian woody vegetation is more abundant now due to human-induced
disturbances than in pre-European times. Thus, some
declines in woody riparian plants currently observed
in the western United States (U.S.) may actually be
resilience in the riparian ecosystem exhibiting slow
recovery to a natural dynamic state.
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EXISTING LITERATURE
Land Use Change
Eventually, channels lose their riffle areas, streams
migrate laterally, pools shallow out, water tables
lower, and riparian vegetation composition shifts from
hydric to more mesic species.
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Invasive Species
The two major invasive plants, which according to
the literature pose a threat to western riparian ecosystems are Russian-olive (Elaeagnus angustifolia), a
native of Europe and western Asia, and tamarisk
(Tamarix chinensis). Both were introduced into the
U.S. in the early 1800s from Europe, and are of concern because of their ability to colonize riparian sites
and exclude native species (Howe and Knopf, 1991).
However, in parts of the West, invasive fish, crustaceans, and mollusks are creating major threats to
native fisheries and the aquatic parts of the riparian
ecosystem (Rinne, 1993).
The life history of the invading species has contributed to their success (Horton, 1977). Tamarisk
and Russian-olive negatively affect native riparian
species germination and establishment in three ways:
(1) the exotic species have a longer period to exploit
suitable germinating conditions, (2) shading by exotics minimizes the potential for successful cottonwood
regeneration, and (3) the exotics are better able to
compete for shade and limited moisture (Howe and
Knopf, 1991) and to tolerate drought (Cleverly et al.,
1997).
Coupled with its ability to increase soil salt concentrations through autogenic processes, tamarisk has a
high salt tolerance that gives it a competitive advantage over many native species (Stromberg et al.,
2004, 2007). Tamarisk combines fast growth in wet
years with extreme tolerance of water and heat stress
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Climate Change
(Cleverly et al., 1997). Reduction in streamflow by
climate change, flow regulation, water diversion, or
dam construction further aggravates the tamarisk
threat. In the Southwest, tamarisk has invaded many
mesquite bosques and cottonwood ⁄ willow forests
along the Rio Grande, the San Pedro, and the lower
Colorado River (Stromberg et al., 2007). In Colorado,
deteriorating riparian cottonwood stands are being
replaced by Russian-olive, raising concerns about the
future ecological status of these important riparian
ecosystems. In New Mexico, the Rio Grande riparian
system will likely be dominated by exotic species
within the next century (Howe and Knopf, 1991).
Reductions in riparian plant diversity, faunal-plant
assemblages, uniqueness, and critical habitat not
only influence terrestrial wildlife and vegetation, but
also affect macroinvertebrates within the stream.
Changes in the quality of stream organic material
inputs together with alterations of the duration,
intensity, and quality of light transmitted to the
aquatic environment, affect the quality, types, and
abundances of substrates for many micro and macroinvertebrates (Obedzinski et al., 2001).
Climate change can influence riparian ecosystems
due to the reliance of these systems on the presence
of water (Naiman et al., 2002). Excessive water
(floods) or insufficient flows (drought) due to climate
change can affect both the biological and physical
compositions of riparian areas (Abrahams et al.,
1995; Dixon et al., 2009). Barnett et al. (2008) attributed many of the observed changes affecting riparian
ecosystems such as river flow, snow packs, temperature, etc. to human activities. Future climate changes
will certainly produce vegetation shifts (Huxman and
Scott, 2007; Chambers and Pellant, 2008) that may
or may not be mitigated with restoration techniques
(Seavy et al., 2009).
As a major feature of climate change, drought creates stress by reducing the ability of plants to photosynthesize, and by limiting the moisture necessary
for germination or the seasonal flooding required by
flood-adapted species such as cottonwood (Obedzinski
et al., 2001). Kranjcec et al. (1998) found that groundwater level declines and the lack of seasonal flooding
had serious effects on cottonwood reproduction and
establishment. Drought also can influence native
riparian species because they cannot compete with
exotic species better adapted to drier conditions and
minimal flooding. Cleverly et al. (1997) and Stromberg et al. (2007) noted that summer flooding, no
flooding, or reduced or altered water tables enabled
tamarisk invasion of cottonwood-willow systems in
Arizona. Johnson et al. (1976) identified the importance of fresh alluvium for the establishment of
pioneer species such as cottonwood (Populus spp.)
and willow (Salix spp.). If this alluvial development
is prevented by drier weather patterns, then species better adapted to nondisturbance, such as oak
(Quercus spp.), might begin to dominate.
Timber Harvesting
Settlement of the West in the 19th Century
resulted in removal of woody riparian vegetation and
the decline of some of these ecosystems. Sands and
Howe (1977) indicated that there were about
300,000 ha of riparian woodlands in California in the
first half of the 19th Century. By the end of that century, these woodlands had been reduced significantly
for fuel wood, fence posts, and building materials,
and converting land to agriculture. By 1977, only
4,725 ha of the 19th Century riparian woodlands
remained intact.
A similar reduction in riparian forests also
occurred in the Southwest. Forest harvesting combined with grazing, conversions to agriculture, and
water regulation reduced riparian woody vegetation
to only 5% of what was originally found at the beginning of the western settlement (Johnson and Haight,
1984).
Logging affects western riparian ecosystems
through tree falling, log skidding, road construction,
and direct removal of vegetation (DeBano and
Schmidt, 1989a,b). The first three factors compact
and disturb soil, which increases erosion, depresses
growth, and further stresses residual vegetation.
Removal of vegetation can alter thermal regimes,
increase soil loss, diminish ecological characteristics
such as structural diversity, alter species composition, and improve site conditions for invasions by
nonnative plants and other biota.
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Recreation
Recreationists are frequently drawn to riparian
areas in the West because of the water resources and
lush vegetation (Winter, 1993). Despite their value to
land managers, recreation activities such as camping,
hiking, and off-road vehicles use are posing threats to
western riparian systems (Aitchison et al., 1977;
Johnson and Carothers, 1982). Heavy recreational
use can result in damage such as (1) reducing density
and diversity of herbaceous plants, (2) lowering tree
and shrub vigor, (3) eliminating seedlings and younger trees, (4) increasing tree diseases, (5) shifting
plant species diversity in favor of disturbanceadapted species, and (6) increasing the potential for
exotic species spread.
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RIPARIAN ECOSYSTEMS
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WESTERN NORTH AMERICA: AN ANALYSIS
OF
EXISTING LITERATURE
from obtaining moisture. In some cases, root systems
of riparian tree species are not able to grow fast
enough to keep up with dropping water table levels
(Groeneveld and Griepentrog, 1985).
In the Southwest, primary, secondary, and tertiary
tributaries of all major rivers have been affected by
water diversions of some type. By the middle of the
20th Century, the effects of water diversion on the
Gila-Salt-Verde River system were so great that
<50% of their original length remained free flowing
(Johnson and Haight, 1984).
Recreation threats are often linked to other threats.
Manning (1979) found that younger trees were
reduced or eliminated on moderately or heavily used
recreational sites. In areas of concentrated use, aspen
stands have shown damage from a variety of recreation-related impacts including bark carving, soil trampling, firewood cutting, and removal of young suckers.
The results of this damage are: (1) reduced or no
advanced regeneration, (2) tree vigor reduction in all
size classes, and (3) increased susceptibility to insect
attack and disease spread. Changes in flow, climate
change, and insect and disease stresses are commonly
entwined with recreation impacts.
Groundwater Depletion
Groundwater depletion is a function of consumptive use by urban and agricultural users, flow regulation, and dam construction. These alterations of
flow regimes have markedly lowered groundwater
levels in some areas. Groundwater is important to
riparian vegetation during periods of drought or in
areas of low seasonal precipitation (Groeneveld and
Griepentrog, 1985; Stromberg et al., 1992). Stress on
mesquite woodlands increased with increasing
groundwater withdrawal from an ephemeral creek
in Arizona (Stromberg et al., 1992), and elsewhere
in the Southwest (Stromberg et al., 2004). Although
summer rains and seasonal high flows temporarily
reduced water stress, the effects of a declining
water table usually are not mitigated and ultimately
lead to continued stress and riparian ecosystem
declines.
Diversions of rivers and groundwater pumping in
alluvial riparian plains for agriculture irrigation
have contributed to water quality problems and
groundwater depletion (Johnson and Haight, 1984;
Johnson and Riley, 1984; National Academy of Science, 2002).
Water Quality – Salinity
Water quality changes in streams, lakes, and
ponds associated with riparian ecosystems are associated with other threats, chiefly agriculture, grazing,
mining, fire, forest harvesting, urbanization, and recreation. While mining, recreation, and urbanization
produce numerous, but localized nonpoint source pollution problems, agriculture, including livestock grazing, probably is the largest threat to riparian
ecosystems due to nutrient release from fertilizers,
bacteria associated with animal fecal material, and
salinization of irrigation return waters (Brown, 1984;
Mueller and Moody, 1984). In the Southwest, salinization of tributaries and the main stem reaches of
the Colorado River has been a major water quality
problem (Johnson and Riley, 1984; Jonez, 1984).
Water Diversion
Dams and the irrigation diversion canals often
associated with dams have significantly altered habitat for woody riparian vegetation in many parts of
the West. Stromberg et al. (2004, 2007) surmised that
the altering of water flows, especially by diversions,
is among the greatest threats to Sonoran cottonwoodwillow ecosystems. Rood et al. (2005, 2007) rated
river damming and associated water diversion right
after livestock grazing and agriculture as primary
causes of decline in cottonwood riparian ecosystems.
Many riparian species are sensitive to changes in
the hydrologic regime that affect flooding periodicities
and water table depth. Cottonwood in particular has
been significantly affected by water diversion. Like
many western riparian species, cottonwoods are
adapted to spring flooding and high water tables.
Water diversion has not only significantly reduced
flooding events, but also has resulted in substantial
lowering of riparian water tables to depths that prevent newly established woody plants like cottonwoods
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Fire
The role of fire in the dynamics of riparian zones
is poorly understood (Dwire and Kauffman, 2003).
Riparian areas along the Gila River in the Southwest periodically burned during dry periods (Baker
et al., 2004). In the Great Plains, tree-dominated
riparian woodlands are somewhat limited by fire
(Boldt et al., 1978). Fire also affects germination
and establishment of many obligate riparian tree
species. Aspen seedling establishment has been
observed to be greatest in riparian zones that had
burned to mineral soil. Fires have a larger effect on
shaping the ecological characteristics of riparian
zones than was believed in the past (Pettit and
Naiman, 2007).
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ASSESSING THE LITERATURE
Past and current research is exploring relationships among native and exotic plants and fire. For
example, Busch and Smith (1995) found that tamarisk had a competitive advantage in water uptake
over willow coppice sprouts on burned sites in the
Southwest. Accumulation of flammable tamarisk leaf
litter also may contribute to the occurrence of episodic fires that destroy native tree species such as
willow (Smith et al., 2009).
Wildland fires have natural sources of ignition but
they are more closely a result of human activities
(Stromberg et al., 2004). Fires can affect the amount
of sediment that reaches streams, alter the loadings
of coarse woody debris into riparian areas, and can
significantly affect the vigor and type of riparian vegetation (Baker et al., 2004). Prescribed fires and wildfires of low-to-moderate severity usually do not cause
much postfire sedimentation. By contrast, severe fires
often produce enough sediment to overload the transport capacity of a stream. While this can provide ecological benefits for aquatic habitats in the short term,
that same debris can cause long-term disruptions of
fluvial, sediment, and biological processes (Davis
et al., 1988; DeBano et al., 1996).
During our literature assessment, we looked at close
to 1,000 articles, reports, books, and chapters, of which
we used 453 for the analysis described here. Literature
was obtained through searches at university libraries,
online search engines such as the U.S. Forest Service
‘‘TreeSearch’’ and Google, as well as by looking at the
reference sections of previously selected papers. Once
we had chosen about 500 publications based on the
criteria listed below, we started our assessment. During our evaluation a number of papers were eliminated
from our consideration for various reasons leaving 453
papers. While this method did not guarantee that all
‘‘important’’ riparian papers were included, we believe
that the sheer volume of publications provides a good
representation of threats to riparian ecosystems in
western North America in the scientific literature. To
include a paper in our study, it needed to describe at
least one threat to a riparian ecosystem in western
North America (U.S., Canada, and Mexico). During
our review we divided publications into five different
categories. We considered a publication a research
paper, which was our main focus, if the author(s) asked
a research question and conducted an experiment,
such as using a cattle exclosure to determine the
impacts of cattle grazing (Bryant, 1982; Roath and
Krueger, 1982; Bohn and Buckhouse, 1985; Schulz and
Leininger, 1990; Allen and Marlow, 1991) or running
numerous simulations to determine the effects of
climate change on riparian ecosystems (Barnett et al.,
2008; Dixon et al., 2009). These papers usually only
address one threat at a time. We regarded a publication as a management paper if it described management action, application or implementation without
experimental design. An example would be a description of stream modification to benefit a specific fish
population and the following results (Andrews et al.,
1985; Wesche, 1985; Konopacky et al., 1986). (This is
how we did it and this is what happened.) A publication illustrating one or more threats to riparian ecosystems summarizing the work of others was labeled a
literature review (Belsky et al., 1999; Malmqvist and
Rundle, 2002; Poff and Zimmerman, 2010). Historical
comparison papers compare a specific riparian site of
at least two points in time usually more than 50 years
apart (i.e., 1880 vs. 1980) and illustrated the differences at each point in time, usually using photographic
comparisons (Betancourt, 1990; Starnes, 1995). The
documented change was then associated with a specific
cause, which is considered a threat to riparian ecosystems as described below. If a publication gave an overview of threats to riparian ecosystems in a broad
discussion or suggested a general direction in which
riparian management should be headed (Chambers,
Mining
Mining has had impacts on riparian ecosystems
throughout the western U.S. The major threats to
riparian ecosystems have come from forest harvesting
to provide mine timbers and fuelwood for underground mines. Placer mining as well as sand and
gravel operations have altered flow and sediment
regimes (Heifner, 1978; Andrews et al., 1985). Mines
have also had drastic impacts on water quality and
altered fish habitats (Martin and Platts, 1981).
Multiple Threats
Riparian ecosystems in the West are being influenced by a variety of stressors. In most cases it is difficult to clearly distinguish the individual impact of
each threat. The effects of an obvious threat might
mask those of a more insidious one. For instance,
Medina (1996) and Neary and Medina (1996) found
that threats to riparian meadows in the White Mountains of Arizona caused by obvious cattle grazing were
masking those already imposed by elk grazing and climate change. Thus, riparian ecosystem threats need to
be examined carefully to determine causes and effect.
In some instances this may not be possible. Furthermore, the presence of multiple threats to a riparian
ecosystem can create a negative synergy, which can
not be explained by any one individual threat.
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THREATS
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RIPARIAN ECOSYSTEMS
IN
WESTERN NORTH AMERICA: AN ANALYSIS
Spatial Distribution of Threats
Next, we looked at where threats to western riparian ecosystems are reported in the existing literature.
The top three states, namely Arizona (22.5%), California (17.4%), and New Mexico (11%), are mentioned in
half of the literature where states or sites within
these states (such as the Grand Canyon National
Park in Arizona) are mentioned (see Table 2). Colorado is a close fourth (10.7%) and together with the
top three most mentioned states is located in the
southwestern part of the country, which as a general
TABLE 1. Threats to Riparian Ecosystems Addressed
in the Literature Are Ranked Here by the Number
of Appearances as Primary Threat and Percentage
Thereof in This Assessment of 453 Publications.
RESULTS
The top four of the 22 threats to riparian ecosystems, are considered the primary threat in about
AMERICAN WATER RESOURCES ASSOCIATION
Rank
Primary
Threat
Addressed
1 (1)
2 (2)
3 (3)
4 (2)
5 (7)
6 (5)
7 (4)
8 (7)
9 (6)
10 (6)
11 (6)
12 (8)
13 (9)
14 (13)
15
16
16
16 (8)
17 (13)
17
17 (9)
17
Grazing
Dams
Land use change
Invasive species
Timber harvest
Climate change
Recreation
Salinity
Water diversion
Groundwater pumping
Fire
Mining
Nonpoint source pollution
Fuel management
Road construction
Fire suppression
Insect and diseases
Woody encroachment
Beavers
Beaver trapping
Parasitism (cowbirds)
Pollution
Number of Percentage of
(research)
(research)
Publications Publications
140
62
54
45
24
20
18
17
14
13
12
11
6
4
3
2
2
2
1
1
1
1
(59)
(27)
(11)
(27)
(3)
(7)
(8)
(3)
(6)
(6)
(6)
(2)
(1)
(1)
(0)
(0)
(0)
(2)
(1)
(0)
(1)
(0)
30.9
13.7
11.9
9.9
5.3
4.4
4.0
3.8
3.1
2.9
2.6
2.4
1.3
0.9
0.7
0.4
0.4
0.4
0.2
0.2
0.2
0.2
(34.5)
(15.8)
(6.4)
(15.8)
(1.8)
(4.1)
(4.7)
(1.8)
(3.5)
(3.5)
(3.5)
(1.2)
(0.6)
(0.6)
(1.2)
(0.6)
Note: The numbers in parentheses refer to the ranking of threats
in research papers and the total number of research papers
addressing these threats.
FIGURE 1. Relative Frequency Distribution of the
Number of Threats Addressed by Each Type of Paper.
OF THE
EXISTING LITERATURE
two-thirds of the publications we assessed (see
Table 1). Grazing (or the improper management of
livestock in riparian zones) alone accounted for
almost one-third (31%), dams were a distant second
(14%), followed closely by land use change (12%).
Invasive species [mostly salt cedar (Tamarisk spp.)
and Russian-olive (E. angustifolia)] ranked fourth
(9.9%), while timber harvest rounded out the top five
with a mere 5.3%. The remaining 17 primary threats,
their total number of occurrences as such in the literature and the corresponding percentages are listed in
Table 1.
2008; Sada, 2008) or what wide-ranging protocols and
policies should be followed (McCormick et al., 2009;
Seavy et al., 2009), it was categorized as a policy
paper.
Of the 453 publications included in our evaluation
we assessed 171 research papers (38%), 98 policy
papers (22%), 86 literature reviews (19%), 71 management papers (16%), and 27 historical comparisons
(6%). The majority of all publications types addressed
only one threat at a time (see Figure 1). With the
exception of the policy paper category a single threat
was addressed about half of the time or more in all
publication types. About three quarters of the
research papers dealt with a single threat. The most
diversity in the number of threats addressed was
found in the policy paper and literature review categories. The maximum number of threats addressed
by any one publication was 12 by one literature
review (to be expected) and one research paper (not
expected – perhaps misclassified).
For our analysis we selected one primary threat for
each paper. This task was simplified since more than
half of the publications assessed herein only addressed
a single threat. Furthermore, the majority of the
papers describing multiple threats tended to focus on a
primary threat and mentioned other threats in their
literature review or discussion. There were only a
handful of publications where a primary threat was
not obvious. Here, we chose the first threat that was
described in detail as the primary threat.
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POFF, KOESTNER, NEARY,
State
Number of
Publications
%
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Arizona
California
New Mexico
Colorado
Oregon
Nevada
Utah
Idaho
Wyoming
Montana
Washington
Texas
South Dakota
Alaska, North Dakota, Oklahoma
Kansas, Missouri, Nebraska
102
79
50
48
44
31
24
22
20
16
14
13
3
2
1
22.5
17.4
11.0
10.6
9.7
6.8
5.3
4.9
4.4
3.5
3.1
2.9
<1
<1
<1
TABLE 3. Publications That Did Not Specify a State but
Rather a Region or General Area, Are Ranked Here by
the Number of Appearances in This Threats Assessment.
General Region
Number of
Publications
Western U.S.
Southwest
Western Canada
Western North America
Pacific Northwest
Colorado River Basin
Intermountain West
Mexico
Great Basin
53
18
11
11
8
7
6
6
5
Rank
1
2
3
3
4
5
6
6
7
region, is referred to by an additional 4% of papers,
when no specific states are given (see Table 3). Arizona, California, New Mexico, and Colorado are also
part of the Colorado River Basin, which is a separate
region in a number of publications.
Research Papers
For further analysis of the existing literature we
focused on publications describing research. The
other categories of papers typically had a tendency to
describe the symptom of degrading riparian ecosystems rather than the causes. Research papers often
focus on the underlying causes of the degradation
and attempt to determine the actual threats to riparian areas. When comparing the primary threats in
the entire body of literature, we also assessed the
main threats in research papers separately (see
JAWRA
HENDERSON
Table 1). It is noticeable that grazing and dams top
both lists, and land use change places third, even
though it makes up twice the percentage of research
publications than in the general literature (12 vs.
6%). Invasive species are also the second highest
ranked threat in the research literature but only the
fourth highest in the general literature with 16 vs.
10%. Timber harvest is considered a threat in over
5% of the general literature, but <2% in the research
literature. Additionally notable is that salinity and
mining received twice the attention in the general
literature they received in the research literature –
percentagewise.
In this assessment, we also looked at the spatial
(Figure 2) and temporal (Figure 3) distribution of the
research literature. When examining the spatial distribution of riparian threats research, several trends
become obvious. For one, the majority of the research
appears to have been conducted in Arizona, the only
state in the nation with a negative water balance
(Fierro and Nyer, 2007). Similar to the distribution of
all publications assessed within this paper, California, New Mexico, Colorado, Oregon, and Nevada,
respectively, make up the top six western states,
where research on riparian threats was conducted.
With the exception of coastal areas of California and
Oregon, these states are located in the arid and semiarid region of the western U.S., where water
resources are scarce. It is also noteworthy that grazing in these states is just one of many threats, unlike
Oregon where it is considered – according to the
research – to be the major threat to riparian areas.
In Colorado and Nevada grazing is the most
researched threat among many threats, but it is far
from dominating the research as is the case in Oregon. In Arizona invasive species and dams and in
California land use change are threats prompting the
most research, while in New Mexico grazing, invasive
species and dams equally share rank number one in
the research literature.
In the temporal analysis there are also several
noteworthy observations. For example, the first
research publication identifying a threat to a riparian
area dates back to the 1930s and addresses invasive
species. This threat has remained in the research
literature throughout the decades, but so far has not
reached the threat ‘‘super-status’’ that grazing
reached in the 1980s, when it dominated almost half
of all the research conducted on riparian threats.
Since then conducting research on grazing as a threat
in riparian zones has become less prevalent, but it
still remains a major concern. Timber harvest, which
was considered a threat in some states in the 1970s
and 1980s, disappeared in the research literature in
the 1990s due to declines in harvesting on Federal
lands, but reappeared this century. Climate change
TABLE 2. The Location of Study Sites in Western States
Represented Throughout the Literature Are Ranked
Here by the Number of Appearances and Percentage
Thereof in This Assessment of 453 Publications.
Rank
AND
8
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THREATS
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RIPARIAN ECOSYSTEMS
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OF
EXISTING LITERATURE
FIGURE 2. This Bar Graph Shows Which Threats Research Addressed in What State and How Many Times.
FIGURE 3. This Bar Graph Shows Temporal Distribution of Research Papers Addressing the Primary Threats to Riparian Ecosystems.
that the only state in the nation that has a negative
water budget, namely Arizona, has the most publications, research and otherwise, addressing the threats
to its riparian zones. In fact, Arizona has lost 95% of
its riparian habitat in the past 150 years. Other western states, California, Colorado, New Mexico, and
Nevada, where water is scarce and ecosystems compete with humans for the precious source of life, publications address the threats to the dwindling
riparian zones. In states with more mesic environments (western Oregon, Washington, Idaho, and
Wyoming), the focus of riparian threat analyses is
geared toward fisheries and aquatic environments.
Other trends being revealed by the spatial analysis
show that where water is scarce the sheer number of
threats is higher than in areas where water is abundant. For example, Arizona has 17 different threats
addressed by research and California has 13, vs.
as a threat to riparian ecosystems is a newcomer to
the scene this century. And so far (2010 was the latest publication included in this assessment) climate
change already ranks as the fifth largest threat being
researched. It is also interesting to note that the
1980s saw a peak of doing research on riparian
threats.
DISCUSSION
So what does it all mean? Assessing the existing
literature for threats to riparian ecosystems revealed
trends, which may not have been so obvious to the
casual observer, but also confirmed some likely
assumptions. It should not be surprising, for example,
JOURNAL
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AMERICAN WATER RESOURCES ASSOCIATION
9
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POFF, KOESTNER, NEARY,
HENDERSON
hydrologic regimes of western riparian ecosystems.
This group includes: grazing, timber harvesting, mining, land use change, woody encroachment, invasive
species, fire, and watershed degradation. There are
also threats, such as salinity, nonpoint source pollution, and elimination of native species that are the
by-product of other threats. The reality of the overwhelming complexity of riparian systems and their
role in the landscape requires an examination of
threats individually for quantitative purposes, but also
collectively and qualitatively to expand our understanding of riparian systems and the sources of their
degradation.
Idaho with 6 and Washington with a mere 2. It is
also noticeable that the research in the wetter states
focuses more on the threat that grazing poses,
whereas in the dry states improper bovine management is just one of many issues faced by managers of
riparian zones.
The temporal analysis indicates that grazing, as a
threat to riparian ecosystem, has been the premier
threat discussed in the literature for close to five decades. While it dominated the research conducted in the
1980s, its popularity as a research topic has been
slowly and steadily declining since. However, it is still
the major threat being assessed in the scientific literature, as different approaches to livestock management
in riparian zones are explored. Invasive species and
the management thereof in riparian zones has been a
steady staple of research throughout the existence of
research on threats to riparian ecosystems. While it
never dominated the research – with exception of the
1930s, when it was the only research conducted on
riparian threats (and the only paper from the 1930s) –
invasive species have been a consistent and persistent
threat throughout. It is also interesting to tease out
that dams were not investigated as a threat to riparian
areas until the 1980s but have been prevalent ever
since. The most interesting threat may be climate
change, which is the latest, in terms of recognized
threats to riparian systems in the research literature.
It may have appeared late, but it climbed the list
quickly and is the fourth most researched threat this
century and sixth biggest threat overall, within less
than a decade. Comparing research-based literature to
the other categories of publication assessed in this
paper also provides some insights. Research tends to
focus more on the causes of the threats to riparian ecosystems, some of which, that is, groundwater pumping
and water diversion, are not as spectacular as let’s say
timber harvesting, which is a popular threat only in
the non-research based literature, while the other two
are not. Also, the scope or number of threats to riparian systems addressed in a particular paper is typically
fewer in the research category due to the constraints of
designing experiments. It should also be noted that
often threats to riparian zones are related or overlapping as far as the type and magnitude of degradation
they cause. For example, the most successful invasive
species infestations are typically in riparian areas
where the hydrologic regime has been significantly
altered. Three of the individual threats covered in the
literature pertain directly to hydrologic alteration
namely dams, groundwater pumping, and water diversion, which combined make up more than one-fifth of
all the threats addressed. Furthermore, all of the
threats due to biotic or abiotic changes in site characteristics (e.g., plant cover loss, ecotype change, sedimentation, soil loss, etc.) are indirectly impacting
JAWRA
AND
SUMMARY AND RECOMMENDATIONS
We assessed and analyzed 453 publications, dominated by research papers, on their content about
threats to riparian ecosystems. The authors believe to
have covered a good representation of the existing literature from the 1930s to 2010. Most publications
found on this topic, covered arid and semiarid regions
(especially within the research paper category). Grazing has been declining in its representation in the literature on threats to riparian ecosystems; however,
is still a concern in most western states. Research
papers that examine the causes of riparian degradation, not only its symptoms, are the most informative
publications as they typically examine a single threat
quantitatively. We recommend that future research
focus on the causes of degradation rather than the
symptoms. If and when causes are understood, management changes that have real positive impacts can
be designed and implemented. The literature covering
threats to riparian ecosystems is vast, providing a
plethora of information for managers, decision makers, and other interested parties. The authors hope
that the assessment provided here will help those
interested in the threats to riparian ecosystems in
western North America to gain a better perspective
in terms of what has been written in the literature.
A complete annotated bibliography of the literature
reviewed, can be found at: http://www.rmrs.nau.edu/
awa/ripthreatbib.
A USDA Forest Service General-Technical Report
is also forthcoming.
ACKNOWLEDGMENTS
The authors thank the Western Wildland Environmental Threat
Assessment Center for the support of this analysis. Their vision
and the guidance of Dr. Terry Shaw was especially helpful.
10
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THREATS
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RIPARIAN ECOSYSTEMS
IN
WESTERN NORTH AMERICA: AN ANALYSIS
Abrahams, A.D., A.J. Parsons, and J. Wainwright, 1995. Effects of
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Barnett, T.P., D.W. Pierce, H.G. Hidalgo, C. Bonfils, B.D. Santer,
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Bohn, C.C. and J.C. Buckhouse, 1985. Some Response of Riparian
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AMERICAN WATER RESOURCES ASSOCIATION
EXISTING LITERATURE
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